The purpose of this study was to examine the efficacy and dose-response relationship of a school-based treatment program for high school students with attention-deficit/hyperactivity disorder (ADHD). Two paraprofessionals provided interventions to 24 students with ADHD randomly assigned to the treatment condition at two public high schools. They met in dyadic coaching sessions during one school year. In addition, parents attended weekly parent meetings and adolescents attended group sessions targeting social functioning in the evenings for 10 weeks in the fall semester. Intent-to-treat analyses showed little statistically significant benefit for the participants; however, effect sizes indicated moderate improvements in parent ratings of inattention, relationships with peers, academic impairment, and family functioning. There was large variability in the dosage of services received across participants, and an analysis of outcomes by dosage suggests large differences in response based on the number of sessions attended. This school-based intervention provides a viable option for educators and school mental health professionals who wish to provide interventions for high school students with ADHD, but further development and evaluation are needed.